somatic mutagenesis and regeneration from protoplasts are well developed [41]. Complementation assays
and dominance relationships can also be studied in sterile mutants by the production, via protoplast fu-
sion, of somatic hybrids that behave as diploids [42].
Techniques for the analysis and manipulation of moss using the tools of molecular biology have ad-
vanced very rapidly in the past 10 years (see later), and with these advances has come increasing recog-
nition of its value as a model organism for basic features of plant development [43]. In no small part this
is due to the discovery that in moss a transgene will, with high frequency, homologously recombine with
its genomic counterpart [7]. To date, the moss Physcomitrella patensis the only land plant for which the
resulting analysis of gene function via “knockout” and other, more subtle manipulations is routinely pos-
sible.
A. Basic Life Cycle and Development Program
Most of the genetic analysis that has been performed on bryophytes has been in the moss Physcomitrella
patens; therefore much of what follows will be based on studies of that organism (see reviews in Refs. 3
and 4). Physcomitrellais a relatively simple moss, but its basic developmental pathways appear to be
quite similar to those of all mosses [44,45]. Other moss species whose development and physiology have
been extensively studied, but for which genetic analyses are less well developed, are Ceratodon pur-
pureusandFunaria hygrometrica[46].
Although mature mosses certainly contain many cell types, in the early stages of the life cycle moss
development is rather simple, involving only a few types of differentiated cells. Following germination
of a haploid spore, the initial growth pattern consists of a two-dimensional filamentous network of cells
called a protonema (Figure 4). In most mosses, including Physcomitrella, the initial protonemal cell type
is the chloronema. These are chloroplast-rich cells about 115 m in length that divide about every 20 hr.
As with many other filamentous systems, growth of a protonemal filament occurs solely at the apical cell.
After three or four cell divisions, an apical chloronemal cell begins to differentiate into a second pro-
tonemal cell type, the caulonema. Caulonemal cells contain fewer chloroplasts, are about 160 m in
length, and divide every 8 hr. Thus, after several days of growth, caulonemal filaments and their deriva-
tives dominate the culture. Essentially all subapical caulonemal cells will divide to produce side branches,
and it is these side branches that can give rise to the next stage of development. Most (~90%) become new
chloronemal filaments, a few give rise to new caulonemata, but about 3% begin more complex two- and
three-dimensional growth and become buds, the bryophyte equivalent of apical meristems. A bud then
develops into a gametophore, which consists primarily of the small leafy shoot that is the most conspic-
uous part of a moss gametophyte. A young Physcomitrellagametophore comprises only a few cell types:
the leaf, which is only one cell thick and contains no vascular tissue; its supporting stem; and the rhizoid,
a filament resembling a caulonemal cell that extends from the base. Other mosses may contain more com-
plex structures and even primitive conducting cells. Figure 5 shows a cell lineage chart of the basic early
developmental pattern for P.patens.
As shown in Figure 5, light, calcium, auxins, and cytokinins are all involved in the control of these
early developmental pathways. Most dramatically, exogenously added cytokinin can cause essentially all
side branch initials to become buds. In addition, abscisic acid is believed to mediate stress responses
[47,48], cyclic adenosine monophosphate (cAMP) and calcium have been implicated in playing impor-
tant roles at the subcellular level, and there have been detailed descriptions of the movements of or-
ganelles prior to and during each cell division (reviewed in Ref. 46).
When gametophores have sufficiently developed and been exposed to the right environmental con-
ditions (commonly, cool temperatures), they are induced to make antheridia and archegonia, the organs
that produce sperm and eggs, respectively. After the presence of water allows the motile sperm to effect
fertilization, the diploid sporophyte grows out of the archegonium that housed the egg. The sporophyte
remains largely dependent on the gametophyte for most of its nutritional requirements, although it ap-
pears that there is at least some metabolic separation between the two stages (cited in Ref. 49). It differ-
entiates a sporangium, or capsule, in which spore mother cells undergo meiosis and generate spores. Very
little is known about the factors influencing the development of moss sporophytes. In Physcomitrellathe
sporophyte is all but invisible; only the mature spore capsule can easily be seen, sitting among the game-
tophores. A review of sexual reproduction in Physcomitrellahas been published [49].
DEVELOPMENTAL GENETICS IN LOWER PLANTS 809